Based on the SOHO joint observing programme for velocity fields in sunspots, we have observed oscillations with a period of 3 minutes in the transition region lines of six sunspots. Significant differences between the sunspots are observed in the amplitudes of the oscillations in O v lambda-629 and N v lambda-lambda-1238, 1242. We investigate the amplitudes and the phase relations between the oscillations in peak line intensity, line-of-sight velocity and line width. The spatial distributions of power in the 3 minute oscillations suggest that the whole transition region above the umbra oscillates. We scrutinize the suggestion that the sunspot oscillations are associated with sunspot plumes. Simultaneous recordings of the transition region lines and the chromosphere Si II lambda-1260 line allow us to compare the transition region oscillations with the 3 minute chromosphere oscillations.